Inorg. Chem. 1997, 36, 19-24
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Bistability and Molecular Switching for Semiquinone and Catechol Complexes of Cobalt. Studies on Redox Isomerism for the Bis(pyridine) Ether Series Co(py2X)(3,6-DBQ)2, X ) O, S, Se, and Te Ok-Sang Jung,*,1a Du Hwan Jo,1a Young-A Lee,1a Brenda J. Conklin,1b and Cortlandt G. Pierpont*,1b Korea Institute of Science and Technology, Cheongryang, Seoul 130-650 Korea, and Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309 ReceiVed October 3, 1996X
Intramolecular electron transfer between CoII(SQ) and CoIII(Cat) species has been investigated for the series of complexes Co(py2X)(3,6-DBQ)2, where 3,6-DBQ are semiquinonate and catecholate forms of 3,6-di-tert-butyl1,2-benzoquinone and py2X is bis(pyridine) ether and its heteroatomic analogs with X ) S, Se, and Te. Transition temperature for Co(III)/Co(II) redox isomerism decreases in steps of approximately 30 K in toluene solution and in steps of 80 K in the solid state for the complexes with X ) S, Se, Te. This appears to be primarily associated with an entropy increase that results from low-energy shifts in vibrational modes with increasing heteroatomic mass. Complexes containing py2O have been isolated at room temperature in two charge distributions, CoII(py2O)(3,6-DBSQ)2 and CoIII(py2O)(3,6-DBSQ)(3,6-DBCat). Crystallographic characterization on both forms of the complex [CoII(py2O)(3,6-DBSQ)2, monoclinic, P21/c, a ) 11.0280(2) Å, b ) 30.2750(9) Å, c ) 12.1120(2) Å, β ) 113.490(2)°, V ) 3708.7(1) Å3, Z ) 4, R ) 0.056; CoIII(py2O)(3,6-DBSQ)(3,6-DBCat), monoclinic, P21/n, a ) 9.882(3) Å, b ) 20.915(5) Å, c ) 17.579(4) Å, β ) 91.57(2)°, V ) 3632(2) Å3, Z ) 4, R ) 0.054] has shown that the py2O ligand adopts a planar structure for the Co(II) isomer that shifts to a folded, nonplanar structure with the smaller Co(III) ion. This structural change is responsible for hysteresis in the Co(III) f Co(II) and Co(II) f Co(III) electron transfer steps in the solid state. Optically induced shifts in charge distribution have been investigated using a low-energy polychromatic light source.
Introduction There is currently active interest in the development of molecular electronic agents for use as photonic optical data storage media. Compounds of specific interest are molecular materials that exchange between stable or metastable states in response to an optical signal creating an addressable memory effect. Most research in this area has focused on conjugated organic molecules that undergo frequency-sensitive reversible bond-forming reactions. Recent reviews by Lehn and Irie have presented an overview of this research.2,3 Reversible transformations of transition metal complexes offer potential for the design of inorganic or organometallic switches. Kahn has reported spin transition complexes of iron(II) that show thermal hysteresis,4 and Gu¨tlich has investigated optically induced spin transitions.5 But, no metal complexes have been found that function with the ease and reversibility of the organic systems. Quinone functionalities appear as components in organic switches, and the coupled redox chemistry of quinones with transition metals may provide the basis for an organo-transition metal switch. Lehn and Harriman have studied a quinonetethered form of Ru(bpy)32+ as a system that may exhibit lightinduced switching, but the charge-separated state that results from Ru(II) f Q electron transfer is short-lived.6 X Abstract published in AdVance ACS Abstracts, December 15, 1996. (1) (a) Korea Institute of Science and Technology. (b) University of Colorado. (2) (a) Gilat, S. L.; Kawai, S. H.; Lehn, J.-M. Chem. Eur. J. 1995, 1, 275. (b) Kawai, S. H.; Gilat, S. L.; Ponsinet, R.; Lehn, J.-M. Chem. Eur. J. 1995, 1, 285. (3) Irie, M. In PhotoreactiVe Materials for Ultrahigh Density Optical Memory; Irie, M., Ed.; Elsevier: Amsterdam, 1994; p 1. (4) Kahn, O.; Kro¨ber, J.; Jay, C. AdV. Mater. 1992, 4, 718. (5) Gu¨tlich, P.; Hauser, A.; Spiering, H. Angew. Chem., Int. Ed. Engl. 1994, 33, 2024.
S0020-1669(96)01214-1 CCC: $14.00
Several years ago we reported the equilibrium between Co(III) and Co(II) redox isomers of a complex containing semiquinonate (SQ) and catecholate (Cat) ligands derived from 3,5di-tert-butyl-1,2-benzoquinone (3,5-DBBQ) (eq 1).7
CoIII(bpy)(3,5-DBSQ)(3,5-DBCat) f CoII(bpy)(3,5-DBSQ)2 (1) In recent research this has been extended to include complexes containing a broad class of N-donor coligands and to complexes prepared with 3,6-DBBQ.8,9 Equilibria occur in separate electron transfer (eq 2) and spin transition (eq 3) steps that
CoII(N-N)(SQ)2 CoIII(N-N)(SQ)(Cat) f 6 0 2 1 (dπ) (dσ) (πQ1) (πQ2) (dπ)6(dσ)1(πQ1)1(πQ2)1
(2)
CoII(N-N)(SQ)2 CoII(N-N)(SQ)2 f (dπ)6(dσ)1(πQ1)1(πQ2)1 (dπ)5(dσ)2(πQ1)1(πQ2)1
(3)
convert low-spin Co(III) to high-spin Co(II) in a process that may be viewed as a charge-transfer-induced spin transition. Equilibrium measurements on Co(bpy)(3,5-DBQ)2 recorded in solution and in the solid state have provided thermodynamic parameters that are in agreement with values for intramolecular (6) Goulle, V.; Harriman, A.; Lehn, J.-M. J. Chem. Soc., Chem. Commun. 1993, 1034. (7) Buchanan, R. M.; Pierpont, C. G. J. Am. Chem. Soc. 1980, 102, 4951. (8) (a) Jung, O.-S.; Pierpont C. G. J. Am. Chem. Soc. 1994, 116, 1127. (b) Jung, O.-S.; Pierpont, C. G. Inorg. Chem. 1994, 33, 2227. (9) (a) Abakumov, G. A.; Cherkasov, V. K.; Bubnov, M. P.; Ellert, O. G.; Dobrokhotova, Z. B.; Zakharov, L. N.; Struchkov, Yu. T. Dokl. Akad. Nauk 1993, 328, 12. (b) Adams, D. M.; Dei, A.; Rheingold, A. L.; Hendrickson, D. N. J. Am. Chem. Soc. 1993, 115, 8221.
© 1997 American Chemical Society
20 Inorganic Chemistry, Vol. 36, No. 1, 1997 enthalpy and entropy changes associated with Co(II)/Co(III) redox reactions.10,11 Population of the antibonding dσ orbital results in a relatively large enthalpic increase that is primarily associated with destabilization of metal-ligand bonds. Associated low-energy shifts in vibrational modes with increases in complex spin degeneracy and structural flexibility give a large positive entropy change that is responsible for the strong temperature dependence of the equilibrium.10 Together, these thermodynamic changes define the transition temperature (Tc) for the equilibrium. The Co(III) redox isomers exhibit intense electronic transitions in the infrared near 4000 cm-1.8 The nature of these transitions has not been established experimentally, but indirect evidence points to an assignment as the Cat f Co(III) charge-transfer band leading to the shift in charge distribution.12 The coupled metal-quinone redox chemistry, the potential for light-induced electron transfer, and the associated changes in spectral, structural, and magnetic properties combine to make the cobalt-quinone complexes uniquely well-suited for optical switching applications. Compounds that have been reported previously tend to show abrupt Co(III)/Co(II) transitions; no complex of the series has yet been found to exhibit reversible transition steps with significant thermal hysteresis. In the present report we describe studies on redox isomerism for the series of compounds prepared with the bis(pyridine) ether coligand and its heteroatomic thio-, seleno-, and telluroether analogs. Experimental Section Materials. 3,6-Di-tert-butyl-1,2-benzoquinone (3,6-DBBQ), 2,2′bis(pyridine) ether (py2O), 2,2′-bis(pyridine) thioether (py2S), 2,2′bis(pyridine) selenoether (py2Se), and 2,2′-bis(pyridine) telluroether (py2Te) were prepared by literature procedures.13-17 Co(py2O)(3,6-DBQ)2. Co2(CO)8 (86 mg, 0.25 mmol) and py2O (86 mg, 0.50 mmol) were combined in 30 mL of toluene under an atmosphere of Ar. The mixture was stirred for 5 min, and 3,6-DBBQ (220 mg, 1.00 mmol) dissolved in 30 mL of toluene was added. The solution was stirred for 2 h at room temperature. Evaporation of the solvent gave the dark green microcrystalline product in 70% yield (235 mg). Dark blue crystals of Co(py2O)(3,6-DBSQ)(3,6-DBCat) suitable for crystallographic characterization were obtained by recrystallization from acetone. Dark green crystals of Co(py2O)(3,6-DBSQ)2 were obtained by recrystallization from toluene. Anal. Calcd for C38H48N2O5Co: C, 67.86; H, 7.04; N, 4.13. Found: C, 67.95; H, 7.20; N, 4.17. Co(py2X)(3,6-DBQ)2, X ) S, Se, Te. Procedures used in the syntheses of these complexes were similar to those described above with equivalent quantities of the thio-, seleno-, and telluroether ligands used in place of py2O. Co(py2S)(3,6-DBQ)2 was obtained in 82% yield. Anal. Calcd for C38H48N2O4SCo: C, 66.29; H, 6.84; N, 3.96. Found: C, 66.36; H, 7.03; N, 4.07. Co(py2Se)(3,6-DBQ)2 and Co(py2Te)(3,6DBQ)2 were obtained in 68% and 62% yield. Recrystallization from acetone gave products as unstable acetone solvates with erratic (10) Pierpont, C. G.; Jung, O.-S. Inorg. Chem. 1995, 34, 4281. (11) (a) Richardson, D. E.; Sharpe, P. Inorg. Chem. 1991, 30, 1412. (b) Richardson, D. E.; Sharpe, P. Inorg. Chem. 1993, 32, 1809. (c) Crawford, P. W.; Schultz, F. A. Inorg. Chem. 1994, 33, 4344. (d) Gao, Y.-D.; Lipkowitz, K. P.; Schultz, F. A. J. Am. Chem. Soc. 1995, 117, 11932. (12) Jung, O.-S.; Pierpont, C. G. J. Am. Chem. Soc. 1994, 116, 2229. (13) Belostotskaya, I. S.; Komissarova, N. L.; Dzhuaryan, E. V.; Ershov, V. V. IzV. Akad. Nauk SSSR 1972, 1594. (14) de Villiers, P. A.; den Hertog, H. J. Recl. TraV. Chim. Pays-Bas 1957, 76, 647. (15) Chachaty, C.; Pappalardo, G. C.; Searlata, G. J. Chem. Soc., Perkin Trans. 2 1976, 1234. (16) Grant, H. G.; Summers, L. A. Z. Naturforsh. 1978, 33B, 118. (17) Dunne, S. J.; Summers, L. A.; von Nagy-Felsobuki, E. I. J. Heterocycl. Chem. 1993, 30, 409.
Jung et al. Table 1. Crystallographic Data for CoIII(py2O)(3,6-DBSQ)(3,6-DBCat) and CoII(py2O)(3,6-DBSQ)2 Co(py2O)(3,6-DBSQ)(3,6-DBCat) Co(py2O)(3,6-DBSQ)2 formula fw color space group a (Å) b (Å) c (Å) β (deg) V (Å3) Z T (°C) λ (Mo KR, Å) Fcalcd (g cm-3) µ (mm-1) R, Rw
C38H48N2O5Co 671.71 dark blue P21/n 9.882(3) 20.915(5) 17.579(4) 91.57(2) 3632(2) 4 20 0.710 73 1.228 0.515 0.054, 0.105a
C38H48N2O5Co 671.71 dark blue-green P21/c 11.0280(2) 30.2750(9) 12.1120(2) 113.490(2) 3708.7(1) 4 22 0.710 69 1.203 0.505 0.056, 0.053b
a R ) Σ||F | - |F ||/Σ||F |; R ) [Σw(|F | - |F |)2/Σw|F |2]1/2. b R o c o w o c o ) Σ||Fo| - |Fc||/Σ|Fo|; Rw ) [Σw(Fo2 - Fc2)2/ΣwFo4]1/2.
elemental analyses, and the composition of these complexes was assigned on the basis of their spectroscopic and magnetic properties. Physical Measurements. Electronic spectra were recorded on a Perkin-Elmer Lambda 9 spectrophotometer equipped with a RMCCyrosystems cryostat. Magnetic measurements were made using a Quantum Design SQUID magnetometer at a field strength of 5 kG. Infrared spectra were recorded on a Perkin-Elmer 1600 FTIR with samples prepared as KBr disks. Crystallographic Structure Determinations. Co(py2O)(3,6DBSQ)(3,6-DBCat). Dark blue crystals of the complex were grown from acetone. Crystals form in the monoclinic crystal system, space group P21/n, in a unit cell of the dimensions given in Table 1. Intensity data were measured on an Enraf Nonius CAD-4 diffractometer within the angular range in Θ of 1.95-25.00°. The Co atom was located on a sharpened Patterson map, and phases generated from the location of the metal gave the positions of other atoms of the structure. Final cycles of refinement converged with discrepancy indices of R ) 0.054 and Rw(F2) ) 0.105. Tables containing a full listing of atom positions, anisotropic displacement parameters, and hydrogen atom locations are available as Supporting Information. Co(py2O)(3,6-DBSQ)2. Dark green crystals were obtained by recrystallization from toluene at room temperature. Crystals form in the monoclinic crystal system, space group P21/c, in a unit cell of the dimensions listed in Table 1. Intensity data were measured on a Rigaku R-AXIS II diffractometer at the Molecular Structure Corporation. Of the 15 563 reflections measured, 5370 were unique (Rint ) 0.046) and used in the structure determination and refinement. The structure was solved using the Fourier methods contained in DIRDIF94. Final cycles of refinement converged with discrepancy indices of R ) 0.056 and Rw ) 0.053. Tables containing a full listing of atom positions, anisotropic displacement parameters, and hydrogen atom locations are available as Supporting Information.
Results Equilibria between cobalt-quinone redox isomers have been found to be extremely sensitive to the properties of nitrogendonor coligands. The solid-state transition temperature of Co(tmeda)(3,6-DBQ)2 is 200 K higher than the related complex prepared with N,N,N′,N′-tetramethylpropylenediamine (tmpda).18 The addition of a single methylene group to the chelate ring of the coligand has a dramatic effect on cobalt-quinone electron transfer. The present investigation has been carried out to study the effects of coligand composition and structure for complexes containing bis(pyridine) ether (I) and its sulfur, selenium, and tellurium analogs. Properties of complexes prepared with the X ) S, Se, and Te series are similar; the py2O complex will be described separately. (18) Jung, O.-S.; Jo, D. H.; Lee, Y.-A; Sohn, Y. S.; Pierpont, C. G. Angew. Chem., Int. Ed. Engl. 1996, 35, 1694.
Redox Studies on Co(py2X)(3,6-DBQ)2
Co(py2X)(3,6-DBQ)2, X ) S, Se, Te Synthetic procedures described in earlier studies have been used to prepare members of the Co(N-N)(3,6-DBQ)2 series with the heteroatomic bis(pyridine) ether ligands. Spectral and magnetic measurements on the three complexes prepared with the thio-, seleno-, and telluroether ligands have been used to monitor the equilibrium between low-spin Co(III) and high-spin Co(II) redox isomers (eq 1). It was anticipated, on the basis of values for atomic radius and electronegativity, that the complexes containing S and Se would be similar, with a small change in Tc for the Te complex. It was surprising to find the dramatic shifts shown in Figure 1. At temperatures below 150 K all three complexes are in the CoIII(py2X)(3,6-DBSQ)(3,6-DBCat) isomeric form with S ) 1/2 magnetic moments due to the radical semiquinone ligand. As sample temperature is increased, shifts to the highspin CoII(py2X)(3,6-DBSQ)2 redox isomer are observed with transition temperatures of 370, 290, and 210 K for the ligands containing S, Se, and Te bridging atoms, respectively. Magnetic moments for the CoII(py2X)(3,6-DBSQ)2 isomers reflect the effects of magnetic exchange between the radical ligands and the paramagnetic S ) 3/2 metal ion. There appears to be a pattern for the S, Se, and Te series that the complexes with higher transition temperatures have the largest magnetic moments for the Co(II) isomers. Differences in the strength of Co-SQ antiferromagnetic exchange contribute to variations in the extent to which high-order spin states are populated. In toluene solution a similar pattern is observed from intensity changes in characteristic spectral bands. In all four cases, the Co(III) redox isomer has an intense transition near 600 nm, while the Co(II) isomer has two overlapped transitions centered near 850 nm (Table 2). The Co(III) isomers also have characteristic absorptions in the 1600-1700 nm region of the NIR and a strong band in the 2400-2600 nm region of the infrared. Temperature dependent changes in the intensity of visible transitions have been used to obtain transition temperatures of 255, 225, and
